The flavor was "fuller." probably because they made it from real roots, instead of chemicals. (but did you know that the oil from the sassafras root is a carcinogen?)
he stayed long enough for a root beer and a couple of conversations, then returned. He can't stop thinking about it. He can't help thinking that it can't have been real -- that it must have been a dream.
he can't stop thinking about what Al is asking him to do.
"Did I want to spend years in the past? No. But I did want to go back. If only to hear how Little Richard sounded when he was still top of the pops. Or get on a Trans World Airlines plane without having to take off my shoes, submit to a full body scan, and go through a metal detector. And I wanted another root beer."